Top 25 Core Java Interview Questions and Answers in 2024

Editorial Team

Core Java Interview Questions and Answers

Do you have an upcoming Java-related interview? You need to review questions touching on Core Java if you do. It is the part of the Java programming language used by developers to create general-purpose applications. Since we are interested in helping you succeed in your interview, we have a few questions on Core Java that you can expect in your interview 

Ensure that you adequately prepare and research before your interview. Do not forget to groom yourself well and give the interviewers a good first impression for physical interviews. We hope that the following questions will see you walking out of the interview room a happy person:

1. What Is Core Java?

Core Java is a basic part of the Java programming language used by developers worldwide to create general-purpose applications. It is also referred to as the J2SE, or the Java standard edition, and follows all the concepts of Object-Oriented programming, popularly known as OOP.

2. Mention The Features Of Core Java

The standard Java edition has several features that make it outstanding. It is simple and therefore easy to learn, which explains why most people prefer it when getting into programming. It is also object-oriented and follows all the object-oriented programming principles. Other features include portability, platform independence, security, robustness, architecture neutrality, and multithreadability.

3. Mention All The Memory Areas In Java Virtual Machines

The Java virtual machine allocates several types of memory areas. However, the main ones are class areas, Heap, Stack, program counter register, and native method stack. The Heap is the official runtime data area that allocates memory to other objects, while the class area holds the per-class structures such as field and method data. The native method stack has all the native methods used in a given application, while the program counter register has the address of JVM instruction under execution. Lastly, the stock stores frames.

4. What Are Jre And Jdk In Java?

JRE stands for Java Runtime Environment, a Java Virtual Machine implementation. It is a combination of software tools that help develop different Java applications. It offers the runtime environment thanks to its collection of libraries and other files needed during runtime.

On the other hand, JDK means Have Development Kit. It provides an environment for developing applications and applets thanks to its variety of tools. It also contains the JRE or the Java Runtime Environment.

5. Define Class Loader In Core Java

Core Java, the standard version of Java, has a virtual machine that branches into classloaders generally used for class file loading. This feature first loads a Java program during its runtime. The Java virtual machine has three class loaders: extension, bootstrap, and system classloaders. The bootstrap classloaders are the extension’s classloaders superclass. It has all the class files of Core Java. The extension classloaders are the child of the former but the parent of the system or application classloaders. It is used to load jar files. Lastly, the system or application classloaders load class files from the classpath.

6. Mention The Different Access Specifiers Found In Java

There are four access specifiers found in Java. These normally play a huge role in defining the access scope of a given variable, class, or method. They include default specifiers, which can only be accessed from the package and where all the methods, classes, and variables are of default scope; private, where methods and variables can only be accessed within the class given that they are defined as private; public where all the classes, variables abs methods can be accessed by any class or method given that they have been defined as public and the class of a similar package can only access similar component package.

7. There Are Static Methods And Variables In Core Java. Mention Their Purpose

There are different types of methods and variables in Java. However, those defined as static normally serve all the objects of a given class. Static, in this case, does not refer to the object but is a part of the class. These variables are often stored in the class area and can be accessed without creating the right object. Static comes in handy whenever there is a need to define methods and variables common to all the class objects.

8. What Do You Understand By An Object-Oriented Paradigm?

There are different paradigms when it comes to programming and the associated languages. Core Java relies on the object-oriented paradigm, based on methods defined in the parent class and objects having data. This paradigm, therefore, enjoys the benefits of reusability and modularity. It has several features: it follows a bottom-up approach when designing programs, implements a real-time approach such as abstraction, and focuses on data and the methods used to access and manipulate the object’s data and concepts such as abstraction and encapsulation.

9. Differentiate An Object Programming Language From An Object-Based Programming Language

There are two main differences between an Object-based language and an object-oriented language. Object-oriented languages are based on all the concepts of OOPs, while object-based languages only follow selective concepts, excluding some such as polymorphism and inheritance. Object-based languages also have in-built objects, while object-oriented languages come devoid of ore-built objects. Object-oriented languages include Java and, in this case, Core Java, while object-based languages include JavaScript.

10. Mention The Different Types Of Constructors Used In Java

There are two main constructors used in the basic edition of Java to create general-purpose applications, i.e., default and parameterized constructors. As the name implies, default instructors are generally used to initialize instance variables with the default values and do not accept any other value. It also plays several roles in object creation and is generally invoked by the compiler if not defined. On the other hand, a parameterized constructor initializes instance variables with respective values. They, therefore, accept arguments, unlike their counterparts.

11. Differentiate Constructors And Methods

There are five main differences between Java contractors and methods. Constructors are mainly used to initialize an object state, while methods expose how a certain object behaves. A constructor doesn’t need a return type, while a method must have its return type. Constructors are invoked implicitly if not defined, while Java methods are invoked explicitly. For Java, the constructors’ names must match the class name, which is not mandatory for methods whose names may or may not match. Lastly, a class without a constructor forces the Java compiler to offer a default constructor, which does not apply to the method.

12. There Are Two Methods In Core Java: Static Or The Class Method And The Instance Method. Can You Please Differentiate Them?

Static or class methods differ from instance methods mainly because the former is declared as static while the latter is not declared as static. One doesn’t need to create objects when calling static methods, while instance methods necessitate the creation of objects. Lastly, it is impossible to access instance methods in the static context, such as static nested class and static method, while instance methods empower the access of static and non-static methods.

13. How Is Hashset Different From The Tree Set?

There are several differences between HashSet and TreeSet in Java and Core Java. As the name implies, the HashSet is often implemented via a hash table while the TreeSet relies on SortedSetinterface with trees for data storage. If you want to perform a search, insert and delete operation, use HashSet as it is usually faster than TreeSet. HashSet also allows the null object, which TreeSet does not permit. Other differences are: HashSet does not use a particular order to maintain objects while TreeSset follows a sorted order, HashNet does not allow heterogeneous objects while TreeSet does, and lastly, two objects are compared using the equals () method in HashSet while TreeSet uses compareTo for the same.

14. Mention All The Differences Between Hashmap And Hashtable

There are seven differences between the HashMap and the Hashtable. The HashMap is non-synchronized, meaning that one needs a proper synchronization code to share it between many threads, while the latter is synchronized and can be shared with multiple threads, given that it is thread-safe. HashMap is faster, uses a fail-fast iterator, and inherits AbstractMap, while Hashtable is slower, uses a non-fail fast enumerator, and inherits the dictionary class. Hashtable does not also permit any null key or value. It is traversed using enumerators and iterators, while HashMap allows one null key and several null values and is generally transversed using an iterator.

15. Do You Think Reflection Is Important In Java?

Reflection plays an important role in Java. This runtime API inspects and changes the behavior of classes, methods, and interfaces. It is beneficial if used well as it allows complete analyses of fields, methods, and classes during runtime. It also comes in handy in creating new objects, getting/setting field values, and calling methods. Furthermore, debuggers rely on refection to analyze private members in different classes.

16. From Your Experience Using Java And Core Java, Is It Possible To Call A Given Class Constructor Inside A Different Constructor?

Yes. One can call a specific class constructor inside another constructor through construction chaining. It can be done within the same class and from the class base. For the first method, you only need the keyword this () for constructors in the same class. However, the keyword super () is used from the base class.

Constructor chaining is aligned to inheritance. The superclass constructor is normally called by the subclass constructor, after which the creation of the subclass’s objects begins when the data members of the superclass are initialized. This generally happens with any number of classes as the constructor consistently calls the chain to the top.

17. What Do You Understand By Double Brace Initialization?

Double Brace Initialization is a common term in Java that comes when two independent processes are combined. In this case, two braces exist, and the first one is used to create anonymous inner classes while the second acts as an initialization block. When the first and second braces are used together, double Brace Initialization happens. In this case, the inner class is often used to create and initialize collections, thereby reducing the code and improving readability.

18. Differentiate Heap And Stack

Heap and Stack are two important memory areas in Java and Core Java. However, they differ in memory, access, memory management, lifetime, and usage. Regarding memory, stack memory only serves one execution thread, while Heap’s memory can cover all the parts of a given application. On access, it is impossible to access Stack’s memory by other threads, while objects found in the Heap offer global access. Heap’s memory management is founded on the generation aligned to each object, while Stack’s memory generally follows LIFO. You should also expect Stack to execute until a given thread ends, while Heap’s lifecycle ends with the execution of the application. Lastly, created objects are normally stored in the Heap, while only local primitives and reference variables are found in Stack.

19. You Mentioned Packages At Some Point In Your Answers. Can You Define Packages In Java?

Packages are a set of related interfaces and classes that are stored together. Developers often use them for easy code modularization and reusability optimization. Other classes can also easily import the code within the classes and reuse them. Therefore, these interfaces and classes have several advantages: they help prevent name clashes, offer easier access control on a line of code, make the location of related classes easier, and store hidden classes that only the packages use.

20. Mention And Define The Main Concepts Of Oops

Object-oriented programming is aligned to several concepts: polymorphism, abstraction, encapsulation, and inheritance. In inheritance, a class acquires another class’s property, while for encapsulation, the data and code are wrapped together as one. Polymorphisms refer to a variable, object, or function taking different forms, while abstraction hides implementation details from users but allows them to enjoy functionality.

21. Java (Core Java) Has A Final Keyword. Can You Mention Its Uses?

Java has several keywords, such as the final keyword. It normally acts as a non-access modifier and can be used in several contexts. First, it can act as a final variable since it is impossible to change the value of a variable that has been used with the final keyword. It can only be used as a final method, given that it is practically impossible to override a method using the inheriting class once it is declared final. Lastly, it can be used as a final class, barring a class from extension by any subclass even though it can extend others.

22. Differentiate The Break And Continue Statements In Core Java

The break statement is commonly used in loop and switch statements, while continue is only used with loop statements. The break statement immediately does away with the switch or innermost enclosing loop while continuing causes the execution of the next loop iteration when nested with a switch. Lastly, the break statement ensures that a loop or switch statement is terminated on execution, while the continue statement makes the loop jump to the next iteration.

23. Super () And This () Are Some Of The Most Common Keywords In Core Java. Tell Us About Their Differences

There are five main differences between this () and super (). The first represents a class’s current instance which the latter also does, just that it strictly deals with the parent/base class. This () keyword is generally used to call the default constructor of a similar class, while the super () keyword calls the default constructor of a parent class. Other differences are: this () offers access to a current class’s methods while super () accesses the methods of the base class, this () has to be included in the first line of a given block while super () is the first line of a block and this () is used for painting a current class’s instance while super () points the superclass instance. The main difference between the two is that super () specifically caters to the parent or base class.

24. What Do You Think Constructor Chaining Is In Java And Core Java?

In construction chaining, one constructor is called from another regarding the current object. It only happens through legacy, where a subclass constructor first invokes the superclass constructor. The number of constructors in this chain is not limited.

25. Why Do You Think Core Java Is Important?

Core Java, the standard data version, has several advantages that users worldwide enjoy. It has superb APIs that make it easy to use, open-source libraries, reliable development tools, and a huge development community behind it.


You don’t have to struggle to answer Core Java-related questions if you are familiar with the standard Java edition. All you need to do is prepare well, sharpen your knowledge, work on your confidence and walk into the interview room. You are going to ace this interview! We wish you all the best.